The present invention relates to the technology to expand sound image positions of respective speakers in stereo sound reproduction.
Two speakers for L-ch and R-ch are provided to the speaker apparatus that can reproduce the sound in stereo. When the electronic equipment to which such speakers are provided is a small-sized device, e.g., mobile terminal, small-sized TV, or the like, or when the case intended for portability or space saving is employed, or the like, an interval between two speakers cannot be set widely. In this case, when an interval between two speakers is narrow in this manner, though a wide spreading sound field can be obtained by the stereo sound reproduction compared to the monaural sound reproduction, a center-spread angle between speaker positions in viewed from a listener becomes narrow, and also the obtained wide spreading sound field becomes narrow.
Therefore, the technology to extend a sound field artificially by applying a sound process even when an interval between two speakers is narrow has been developed. For example, in Patent Literature 1, the technology to add a delayed signal obtained by delaying a signal on one channel to a signal on the other channel is disclosed. Also, in Patent Literature 2, the technology using HRTF (Head-Related Transfer Function) is disclosed.    [Patent Literature 1] JP-A-10-28097    [Patent Literature 2] JP-A-09-114479
In the technology disclosed in Patent Literature 1, sound images can be expanded, but localization of sounds is lost because such sound images expand in a blurred fashion. In Patent Literature 2, the process such as the FIR (Finite Impulse Response) filter, or the like is needed, and also a huge amount of process is needed. Also, the localization of sounds can be created precisely by using the HRTF, nevertheless in some cases unnatural localization of sounds is created depending on the listener because a shape of the listener's head is different individually.